Newer computer screen conformations and smaller viewing devices are necessitating more flexible/fluid Markup & CSS (style sheet) authoring these days. Of particular interest is the use of cell phones to browse web pages. The Opera Web pages, Opera Mobile Browser, and Small Screen Rendering Technology are excellent references, the latter one being especially valuable for Windows O/S Web authors. The Apple IPhone uses a mini Web Browser by Safari.
Hand-held mobile devices such as the Blackberry (8800, Curve, Pearl), and the Palm Treo 650, employ Web Page Small Screen Rendering which can be emulated in Firefox via the top drop-down menu: Tools> Web Developer> Miscellaneous> Small Screen Rendering (or via Miscellaneous> Small Screen Rendering from the drop-down menu in Chris Pederick's Web Developer add-on). The same facility is available in Opera, another excellent developer tool, via View> Small screen (toggle) from the top drop-down menu.
Following are my own experiences:
While browsing our local shopping mall I frequently stop at the some of the numerous cell phone vendor kiosks and ask salesman to check how a few of my web pages render in various products -- I haven't been turned down so far -- they are often bored and only too willing to demonstrate their wares (besides there may be a sales possibility).
Quick informal surveys of my web page user populations have shown that cell phone usage of my pages is quite spotty. Only three users reported viewing my Italic Handwriting pages in their cell phones and then only out of curiosity. On the other hand, my Roman Coins pages are visited quite often by enthusiasts at coin shows (using cell phones) wishing to attribute or authenticate coins they are contemplating purchasing.